The present invention relates to catalytic autothermal oxidation processes. More particularly, the present invention relates to a method of regenerating a catalyst used in the catalytic partial oxidation of paraffinic hydrocarbons, such as ethane, propane, and naphtha, to olefins, such as ethylene and propylene.
Olefins find widespread utility in industrial organic chemistry. Ethylene is needed for the preparation of important polymers, such as polyethylene, vinyl plastics, and ethylene-propylene rubbers, and important basic chemicals, such as ethylene oxide, styrene, acetaldehyde, ethyl acetate, and dichloro-ethane. Propylene is needed for the preparation of polypropylene plastics, ethylene-propylene rubbers, and important basic chemicals, such as propylene oxide, cumene, and acrolein. Isobutylene is needed for the preparation of methyl tertiary butyl ether. Long chain mono-olefins find utility in the manufacture of linear alkylated benzene sulfonates, which are used in the detergent industry.
Low molecular weight olefins, such as ethylene, propylene, and butylene, are produced almost exclusively by thermal cracking (pyrolysis/steam cracking) of alkanes at elevated temperatures. An ethylene plant, for example, typically achieves an ethylene selectivity of about 85 percent calculated on a carbon atom basis at an ethane conversion of about 60 mole percent. Undesired coproducts are recycled to the shell side of the cracking furnace to be burned, so as to produce the heat necessary for the process. Disadvantageously, thermal cracking processes for olefin production are highly endothermic. Accordingly, these processes require the construction and maintenance of large, capital intensive, and complex cracking furnaces. The heat required to operate these furnaces at a temperature of about 900.degree. C. is frequently obtained from the combustion of methane which disadvantageously produces undesirable quantities of carbon dioxide. As a further disadvantage, the crackers must be shut down periodically to remove coke deposits on the inside of the cracking coils.
Catalytic processes are known wherein paraffinic hydrocarbons are oxidatively dehydrogenated to form mono-olefins. In these processes, a paraffinic hydrocarbon is contacted with oxygen in the presence of a catalyst consisting of a platinum group metal or mixture thereof deposited on a ceramic monolith support, typically in the form of a honeycomb or foam. Optionally, hydrogen may be a component of the feed. The catalyst, prepared using conventional techniques, is uniformly loaded throughout the support. The process can be conducted under autothermal reaction conditions wherein the feed is partially combusted, and the heat produced during combustion drives the endothermic cracking processes. Consequently, under autothermal process conditions there is no external heat source required; however, the catalyst is required to support combustion above the normal fuel-rich limit of flammability. Representative references disclosing this type of process include the following U.S. Pat. Nos.: 4,940,826; 5,105,052; 5,382,741; and 5,625,111. Disadvantageously, substantial amounts of deep oxidation products, such as carbon monoxide and carbon dioxide, are produced, and the selectivity to olefins remains too low when compared with thermal cracking. Moreover, the references are silent with respect to a method of regenerating the catalyst.
M. Huff and L. D. Schmidt disclose in the Journal of Physical Chemistry, 97, 1993, 11,815, the production of ethylene from ethane in the presence of air or oxygen under autothermal conditions over alumina foam monoliths coated with platinum, rhodium, or palladium. A similar article by M. Huff and L. D. Schmidt in the Journal of Catalysis, 149, 1994, 127-141, discloses the autothermal production of olefins from propane and butane by oxidative dehydrogenation and cracking in air or oxygen over platinum and rhodium coated alumina foam monoliths. Again, the olefin activity achieved in these processes could be improved. The references are also silent with respect to a method of regenerating the catalyst.
U.S. Pat. No. 5,639,929 teaches an autothermal process for the oxidative dehydrogenation of C.sub.2 -C.sub.6 alkanes with an oxygen-containing gas in a fluidized catalyst bed of platinum, rhodium, nickel, or platinum-gold supported on alpha alumina or zirconia. Ethane produces ethylene, while higher olefins produce ethylene, propylene, and isobutylene. Again, the olefin selectivity could be improved, and the reference is silent with respect to a method of regenerating the catalyst.
C. Yokoyama, S. S. Bharadwaj and L. D. Schmidt disclose in Catalysis Letters, 38, 1996, 181-188, the oxidative dehydrogenation of ethane to ethylene under autothermal reaction conditions in the presence of a bimetallic catalyst comprising platinum and a second metal selected from tin, copper, silver, magnesium, cerium, lanthanum, nickel, cobalt, and gold supported on a ceramic foam monolith. The use of a catalyst comprising platinum with tin and/or copper results in an improved olefin selectivity; however, over time at high operating temperatures the second metal vaporizes off the catalyst and catalytic activity decreases. When this occurs the reactor must be shut down to replace or regenerate the catalyst.
In view of the above, it would be desirable to discover an autothermal catalytic process of oxidizing a paraffinic hydrocarbon to an olefin wherein the catalyst can be readily regenerated. Such a process would provide the benefits of catalytic autothermal processes, such as low levels of catalyst coking and simplified engineering, with the added benefit of easy catalyst regenerability. It would be even more desirable if a catalytic autothermal process providing easy catalyst regenerability was to achieve a paraffinic hydrocarbon conversion and an olefin selectivity comparable to those achieved by commercial thermal cracking processes.